1 ffmpeg命令行
一般对视频文件的裁剪 我们通过一行 ffmpeg命令行即可实现，比如
ffmpeg -ss 0.5 - t 3 - i a.mp4 vcodec copy b.mp4
其中 -ss 放置较前 开启精准seek定位 对于mp4而言 seek将从moov中相关索引表查找 0.5s时刻附近最近的关键帧 （此描述可能不精确）
ffmpeg根据此命令是如何裁剪呢？
1.1 命令行入口
执行代码位置
fftools/ffmpeg.c
cpp
int main(int argc, char **argv)
{
int i, ret;
BenchmarkTimeStamps ti;
init_dynload();
register_exit(ffmpeg_cleanup);
setvbuf(stderr,NULL,_IONBF,0); /* win32 runtime needs this */
av_log_set_flags(AV_LOG_SKIP_REPEATED);
parse_loglevel(argc, argv, options);
if(argc>1 && !strcmp(argv[1], "-d")){
run_as_daemon=1;
av_log_set_callback(log_callback_null);
argc--;
argv++;
}
#if CONFIG_AVDEVICE
avdevice_register_all();
#endif
avformat_network_init();
show_banner(argc, argv, options);
/* parse options and open all input/output files */
ret = ffmpeg_parse_options(argc, argv);
if (ret < 0)
exit_program(1);
if (nb_output_files <= 0 && nb_input_files == 0) {
show_usage();
av_log(NULL, AV_LOG_WARNING, "Use -h to get full help or, even better, run 'man %s'\n", program_name);
exit_program(1);
}
/* file converter / grab */
if (nb_output_files <= 0) {
av_log(NULL, AV_LOG_FATAL, "At least one output file must be specified\n");
exit_program(1);
}
for (i = 0; i < nb_output_files; i++) {
if (strcmp(output_files[i]->ctx->oformat->name, "rtp"))
want_sdp = 0;
}
current_time = ti = get_benchmark_time_stamps();
if (transcode() < 0)
exit_program(1);
if (do_benchmark) {
int64_t utime, stime, rtime;
current_time = get_benchmark_time_stamps();
utime = current_time.user_usec - ti.user_usec;
stime = current_time.sys_usec - ti.sys_usec;
rtime = current_time.real_usec - ti.real_usec;
av_log(NULL, AV_LOG_INFO,
"bench: utime=%0.3fs stime=%0.3fs rtime=%0.3fs\n",
utime / 1000000.0, stime / 1000000.0, rtime / 1000000.0);
}
av_log(NULL, AV_LOG_DEBUG, "%"PRIu64" frames successfully decoded, %"PRIu64" decoding errors\n",
decode_error_stat[0], decode_error_stat[1]);
if ((decode_error_stat[0] + decode_error_stat[1]) * max_error_rate < decode_error_stat[1])
exit_program(69);
exit_program(received_nb_signals ? 255 : main_return_code);
return main_return_code;
}1.2 执行流程
代码流程比较清晰 主要流程如下：
1.3 输入上下文设置
ffmpeg将解析的参数存放到OptionParseContext中包含输入及输出参数设置，然后进入输入上下文
根据之前解析的数据OptionPareseContext作为参数传入，确定好seek点，然后调用 avformat_seek_file
cpp
static int open_input_file(OptionsContext *o, const char *filename)
{
InputFile *f;
AVFormatContext *ic;
AVInputFormat *file_iformat = NULL;
int err, i, ret;
int64_t timestamp;
AVDictionary *unused_opts = NULL;
AVDictionaryEntry *e = NULL;
char * video_codec_name = NULL;
char * audio_codec_name = NULL;
char *subtitle_codec_name = NULL;
char * data_codec_name = NULL;
int scan_all_pmts_set = 0;
if (o->stop_time != INT64_MAX && o->recording_time != INT64_MAX) {
o->stop_time = INT64_MAX;
av_log(NULL, AV_LOG_WARNING, "-t and -to cannot be used together; using -t.\n");
}
if (o->stop_time != INT64_MAX && o->recording_time == INT64_MAX) {
int64_t start_time = o->start_time == AV_NOPTS_VALUE ? 0 : o->start_time;
if (o->stop_time <= start_time) {
av_log(NULL, AV_LOG_ERROR, "-to value smaller than -ss; aborting.\n");
exit_program(1);
} else {
o->recording_time = o->stop_time - start_time;
}
}
if (o->format) {
if (!(file_iformat = av_find_input_format(o->format))) {
av_log(NULL, AV_LOG_FATAL, "Unknown input format: '%s'\n", o->format);
exit_program(1);
}
}
if (!strcmp(filename, "-"))
filename = "pipe:";
stdin_interaction &= strncmp(filename, "pipe:", 5) &&
strcmp(filename, "/dev/stdin");
/* get default parameters from command line */
ic = avformat_alloc_context();
if (!ic) {
print_error(filename, AVERROR(ENOMEM));
exit_program(1);
}
if (o->nb_audio_sample_rate) {
av_dict_set_int(&o->g->format_opts, "sample_rate", o->audio_sample_rate[o->nb_audio_sample_rate - 1].u.i, 0);
}
if (o->nb_audio_channels) {
/* because we set audio_channels based on both the "ac" and
* "channel_layout" options, we need to check that the specified
* demuxer actually has the "channels" option before setting it */
if (file_iformat && file_iformat->priv_class &&
av_opt_find(&file_iformat->priv_class, "channels", NULL, 0,
AV_OPT_SEARCH_FAKE_OBJ)) {
av_dict_set_int(&o->g->format_opts, "channels", o->audio_channels[o->nb_audio_channels - 1].u.i, 0);
}
}
if (o->nb_frame_rates) {
/* set the format-level framerate option;
* this is important for video grabbers, e.g. x11 */
if (file_iformat && file_iformat->priv_class &&
av_opt_find(&file_iformat->priv_class, "framerate", NULL, 0,
AV_OPT_SEARCH_FAKE_OBJ)) {
av_dict_set(&o->g->format_opts, "framerate",
o->frame_rates[o->nb_frame_rates - 1].u.str, 0);
}
}
if (o->nb_frame_sizes) {
av_dict_set(&o->g->format_opts, "video_size", o->frame_sizes[o->nb_frame_sizes - 1].u.str, 0);
}
if (o->nb_frame_pix_fmts)
av_dict_set(&o->g->format_opts, "pixel_format", o->frame_pix_fmts[o->nb_frame_pix_fmts - 1].u.str, 0);
MATCH_PER_TYPE_OPT(codec_names, str, video_codec_name, ic, "v");
MATCH_PER_TYPE_OPT(codec_names, str, audio_codec_name, ic, "a");
MATCH_PER_TYPE_OPT(codec_names, str, subtitle_codec_name, ic, "s");
MATCH_PER_TYPE_OPT(codec_names, str, data_codec_name, ic, "d");
if (video_codec_name)
ic->video_codec = find_codec_or_die(video_codec_name , AVMEDIA_TYPE_VIDEO , 0);
if (audio_codec_name)
ic->audio_codec = find_codec_or_die(audio_codec_name , AVMEDIA_TYPE_AUDIO , 0);
if (subtitle_codec_name)
ic->subtitle_codec = find_codec_or_die(subtitle_codec_name, AVMEDIA_TYPE_SUBTITLE, 0);
if (data_codec_name)
ic->data_codec = find_codec_or_die(data_codec_name , AVMEDIA_TYPE_DATA , 0);
ic->video_codec_id = video_codec_name ? ic->video_codec->id : AV_CODEC_ID_NONE;
ic->audio_codec_id = audio_codec_name ? ic->audio_codec->id : AV_CODEC_ID_NONE;
ic->subtitle_codec_id = subtitle_codec_name ? ic->subtitle_codec->id : AV_CODEC_ID_NONE;
ic->data_codec_id = data_codec_name ? ic->data_codec->id : AV_CODEC_ID_NONE;
ic->flags |= AVFMT_FLAG_NONBLOCK;
if (o->bitexact)
ic->flags |= AVFMT_FLAG_BITEXACT;
ic->interrupt_callback = int_cb;
if (!av_dict_get(o->g->format_opts, "scan_all_pmts", NULL, AV_DICT_MATCH_CASE)) {
av_dict_set(&o->g->format_opts, "scan_all_pmts", "1", AV_DICT_DONT_OVERWRITE);
scan_all_pmts_set = 1;
}
/* open the input file with generic avformat function */
err = avformat_open_input(&ic, filename, file_iformat, &o->g->format_opts);
if (err < 0) {
print_error(filename, err);
if (err == AVERROR_PROTOCOL_NOT_FOUND)
av_log(NULL, AV_LOG_ERROR, "Did you mean file:%s?\n", filename);
exit_program(1);
}
if (scan_all_pmts_set)
av_dict_set(&o->g->format_opts, "scan_all_pmts", NULL, AV_DICT_MATCH_CASE);
remove_avoptions(&o->g->format_opts, o->g->codec_opts);
assert_avoptions(o->g->format_opts);
/* apply forced codec ids */
for (i = 0; i < ic->nb_streams; i++)
choose_decoder(o, ic, ic->streams[i]);
if (find_stream_info) {
AVDictionary **opts = setup_find_stream_info_opts(ic, o->g->codec_opts);
int orig_nb_streams = ic->nb_streams;
/* If not enough info to get the stream parameters, we decode the
first frames to get it. (used in mpeg case for example) */
ret = avformat_find_stream_info(ic, opts);
for (i = 0; i < orig_nb_streams; i++)
av_dict_free(&opts[i]);
av_freep(&opts);
if (ret < 0) {
av_log(NULL, AV_LOG_FATAL, "%s: could not find codec parameters\n", filename);
if (ic->nb_streams == 0) {
avformat_close_input(&ic);
exit_program(1);
}
}
}
if (o->start_time != AV_NOPTS_VALUE && o->start_time_eof != AV_NOPTS_VALUE) {
av_log(NULL, AV_LOG_WARNING, "Cannot use -ss and -sseof both, using -ss for %s\n", filename);
o->start_time_eof = AV_NOPTS_VALUE;
}
if (o->start_time_eof != AV_NOPTS_VALUE) {
if (o->start_time_eof >= 0) {
av_log(NULL, AV_LOG_ERROR, "-sseof value must be negative; aborting\n");
exit_program(1);
}
if (ic->duration > 0) {
o->start_time = o->start_time_eof + ic->duration;
if (o->start_time < 0) {
av_log(NULL, AV_LOG_WARNING, "-sseof value seeks to before start of file %s; ignored\n", filename);
o->start_time = AV_NOPTS_VALUE;
}
} else
av_log(NULL, AV_LOG_WARNING, "Cannot use -sseof, duration of %s not known\n", filename);
}
timestamp = (o->start_time == AV_NOPTS_VALUE) ? 0 : o->start_time;
/* add the stream start time */
if (!o->seek_timestamp && ic->start_time != AV_NOPTS_VALUE)
timestamp += ic->start_time;
/* if seeking requested, we execute it */
if (o->start_time != AV_NOPTS_VALUE) {
int64_t seek_timestamp = timestamp;
if (!(ic->iformat->flags & AVFMT_SEEK_TO_PTS)) {
int dts_heuristic = 0;
for (i=0; i<ic->nb_streams; i++) {
const AVCodecParameters *par = ic->streams[i]->codecpar;
if (par->video_delay) {
dts_heuristic = 1;
break;
}
}
if (dts_heuristic) {
seek_timestamp -= 3*AV_TIME_BASE / 23;
}
}
ret = avformat_seek_file(ic, -1, INT64_MIN, seek_timestamp, seek_timestamp, 0);
if (ret < 0) {
av_log(NULL, AV_LOG_WARNING, "%s: could not seek to position %0.3f\n",
filename, (double)timestamp / AV_TIME_BASE);
}
}
/* update the current parameters so that they match the one of the input stream */
add_input_streams(o, ic);
/* dump the file content */
av_dump_format(ic, nb_input_files, filename, 0);
GROW_ARRAY(input_files, nb_input_files);
f = av_mallocz(sizeof(*f));
if (!f)
exit_program(1);
input_files[nb_input_files - 1] = f;
f->ctx = ic;
f->ist_index = nb_input_streams - ic->nb_streams;
f->start_time = o->start_time;
f->recording_time = o->recording_time;
f->input_ts_offset = o->input_ts_offset;
f->ts_offset = o->input_ts_offset - (copy_ts ? (start_at_zero && ic->start_time != AV_NOPTS_VALUE ? ic->start_time : 0) : timestamp);
f->nb_streams = ic->nb_streams;
f->rate_emu = o->rate_emu;
f->accurate_seek = o->accurate_seek;
f->loop = o->loop;
f->duration = 0;
f->time_base = (AVRational){ 1, 1 };
#if HAVE_THREADS
f->thread_queue_size = o->thread_queue_size > 0 ? o->thread_queue_size : 8;
#endif
/* check if all codec options have been used */
unused_opts = strip_specifiers(o->g->codec_opts);
for (i = f->ist_index; i < nb_input_streams; i++) {
e = NULL;
while ((e = av_dict_get(input_streams[i]->decoder_opts, "", e,
AV_DICT_IGNORE_SUFFIX)))
av_dict_set(&unused_opts, e->key, NULL, 0);
}
e = NULL;
while ((e = av_dict_get(unused_opts, "", e, AV_DICT_IGNORE_SUFFIX))) {
const AVClass *class = avcodec_get_class();
const AVOption *option = av_opt_find(&class, e->key, NULL, 0,
AV_OPT_SEARCH_CHILDREN | AV_OPT_SEARCH_FAKE_OBJ);
const AVClass *fclass = avformat_get_class();
const AVOption *foption = av_opt_find(&fclass, e->key, NULL, 0,
AV_OPT_SEARCH_CHILDREN | AV_OPT_SEARCH_FAKE_OBJ);
if (!option || foption)
continue;
if (!(option->flags & AV_OPT_FLAG_DECODING_PARAM)) {
av_log(NULL, AV_LOG_ERROR, "Codec AVOption %s (%s) specified for "
"input file #%d (%s) is not a decoding option.\n", e->key,
option->help ? option->help : "", nb_input_files - 1,
filename);
exit_program(1);
}
av_log(NULL, AV_LOG_WARNING, "Codec AVOption %s (%s) specified for "
"input file #%d (%s) has not been used for any stream. The most "
"likely reason is either wrong type (e.g. a video option with "
"no video streams) or that it is a private option of some decoder "
"which was not actually used for any stream.\n", e->key,
option->help ? option->help : "", nb_input_files - 1, filename);
}
av_dict_free(&unused_opts);
for (i = 0; i < o->nb_dump_attachment; i++) {
int j;
for (j = 0; j < ic->nb_streams; j++) {
AVStream *st = ic->streams[j];
if (check_stream_specifier(ic, st, o->dump_attachment[i].specifier) == 1)
dump_attachment(st, o->dump_attachment[i].u.str);
}
}
input_stream_potentially_available = 1;
return 0;
}1.4 avformat_seek_file
cpp
int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts,
int64_t ts, int64_t max_ts, int flags)
{
if (min_ts > ts || max_ts < ts)
return -1;
if (stream_index < -1 || stream_index >= (int)s->nb_streams)
return AVERROR(EINVAL);
if (s->seek2any>0)
flags |= AVSEEK_FLAG_ANY;
flags &= ~AVSEEK_FLAG_BACKWARD;
if (s->iformat->read_seek2) {
int ret;
ff_read_frame_flush(s);
if (stream_index == -1 && s->nb_streams == 1) {
AVRational time_base = s->streams[0]->time_base;
ts = av_rescale_q(ts, AV_TIME_BASE_Q, time_base);
min_ts = av_rescale_rnd(min_ts, time_base.den,
time_base.num * (int64_t)AV_TIME_BASE,
AV_ROUND_UP | AV_ROUND_PASS_MINMAX);
max_ts = av_rescale_rnd(max_ts, time_base.den,
time_base.num * (int64_t)AV_TIME_BASE,
AV_ROUND_DOWN | AV_ROUND_PASS_MINMAX);
stream_index = 0;
}
ret = s->iformat->read_seek2(s, stream_index, min_ts,
ts, max_ts, flags);
if (ret >= 0)
ret = avformat_queue_attached_pictures(s);
return ret;
}
if (s->iformat->read_timestamp) {
// try to seek via read_timestamp()
}
// Fall back on old API if new is not implemented but old is.
// Note the old API has somewhat different semantics.
if (s->iformat->read_seek || 1) {
int dir = (ts - (uint64_t)min_ts > (uint64_t)max_ts - ts ? AVSEEK_FLAG_BACKWARD : 0);
int ret = av_seek_frame(s, stream_index, ts, flags | dir);
if (ret<0 && ts != min_ts && max_ts != ts) {
ret = av_seek_frame(s, stream_index, dir ? max_ts : min_ts, flags | dir);
if (ret >= 0)
ret = av_seek_frame(s, stream_index, ts, flags | (dir^AVSEEK_FLAG_BACKWARD));
}
return ret;
}
// try some generic seek like seek_frame_generic() but with new ts semantics
return -1; //unreachable
}ffmpeg中大部分fromat都定义实现了read_seek ,一般情况下都会调用av_seek_frame，因为输入文件是mp4格式 .format对应 mov ,
av_ssek_frame->seek_frame_internal-> read_seek ->mov_read_seek
cpp
AVInputFormat ff_mov_demuxer = {
.name = "mov,mp4,m4a,3gp,3g2,mj2",
.long_name = NULL_IF_CONFIG_SMALL("QuickTime / MOV"),
.priv_class = &mov_class,
.priv_data_size = sizeof(MOVContext),
.extensions = "mov,mp4,m4a,3gp,3g2,mj2",
.read_probe = mov_probe,
.read_header = mov_read_header,
.read_packet = mov_read_packet,
.read_close = mov_read_close,
.read_seek = mov_read_seek,
.flags = AVFMT_NO_BYTE_SEEK | AVFMT_SEEK_TO_PTS,
};真正确定seek位置代码函数 ff_index_search_timestamp flags不同 查询规则有所不同
cpp
int ff_index_search_timestamp(const AVIndexEntry *entries, int nb_entries,
int64_t wanted_timestamp, int flags)
{
int a, b, m;
int64_t timestamp;
a = -1;
b = nb_entries;
// Optimize appending index entries at the end.
if (b && entries[b - 1].timestamp < wanted_timestamp)
a = b - 1;
while (b - a > 1) {
m = (a + b) >> 1;
// Search for the next non-discarded packet.
while ((entries[m].flags & AVINDEX_DISCARD_FRAME) && m < b && m < nb_entries - 1) {
m++;
if (m == b && entries[m].timestamp >= wanted_timestamp) {
m = b - 1;
break;
}
}
timestamp = entries[m].timestamp;
if (timestamp >= wanted_timestamp)
b = m;
if (timestamp <= wanted_timestamp)
a = m;
}
m = (flags & AVSEEK_FLAG_BACKWARD) ? a : b;
if (!(flags & AVSEEK_FLAG_ANY))
while (m >= 0 && m < nb_entries &&
!(entries[m].flags & AVINDEX_KEYFRAME))
m += (flags & AVSEEK_FLAG_BACKWARD) ? -1 : 1;
if (m == nb_entries)
return -1;
return m;
}flags:
#define AVSEEK_FLAG_BACKGROUND 1 ///<<Seek Background 往后移，
#define AVSEEK_FALG_BYTE ///<<<seeking based on position in bytes 让时间戳
变成一个 byte, 按照文件的大小位置跳到那个位置
#define AVSEEK_FLAG_ANY ///<<<seek to any frame, even non-keyframes // 移动到任意帧的位置，不去找前面的关键帧，
#define AVSEEK_FLAG_FRAME ///<<<seeking based on frame number // 找关键帧，一般与 AVSEEK_FLAG_BACKGROUND 一起使用
2 TS
对于ts切割格式采用同样的方式分析 mpegts.c , 并未定义 read_seek
cpp
AVInputFormat ff_mpegts_demuxer = {
.name = "mpegts",
.long_name = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
.priv_data_size = sizeof(MpegTSContext),
.read_probe = mpegts_probe,
.read_header = mpegts_read_header,
.read_packet = mpegts_read_packet,
.read_close = mpegts_read_close,
.read_timestamp = mpegts_get_dts,
.flags = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
.priv_class = &mpegts_class,
};从而进入 ff_seek_frame_binary 方法，由于是实时流没有index_entry 调用ff_gen_search 生成seek pos gen_seek
cpp
int ff_seek_frame_binary(AVFormatContext *s, int stream_index,
int64_t target_ts, int flags)
{
const AVInputFormat *avif = s->iformat;
int64_t av_uninit(pos_min), av_uninit(pos_max), pos, pos_limit;
int64_t ts_min, ts_max, ts;
int index;
int64_t ret;
AVStream *st;
if (stream_index < 0)
return -1;
av_log(s, AV_LOG_TRACE, "read_seek: %d %s\n", stream_index, av_ts2str(target_ts));
ts_max =
ts_min = AV_NOPTS_VALUE;
pos_limit = -1; // GCC falsely says it may be uninitialized.
st = s->streams[stream_index];
if (st->index_entries) {
AVIndexEntry *e;
/* FIXME: Whole function must be checked for non-keyframe entries in
* index case, especially read_timestamp(). */
index = av_index_search_timestamp(st, target_ts,
flags | AVSEEK_FLAG_BACKWARD);
index = FFMAX(index, 0);
e = &st->index_entries[index];
if (e->timestamp <= target_ts || e->pos == e->min_distance) {
pos_min = e->pos;
ts_min = e->timestamp;
av_log(s, AV_LOG_TRACE, "using cached pos_min=0x%"PRIx64" dts_min=%s\n",
pos_min, av_ts2str(ts_min));
} else {
av_assert1(index == 0);
}
index = av_index_search_timestamp(st, target_ts,
flags & ~AVSEEK_FLAG_BACKWARD);
av_assert0(index < st->nb_index_entries);
if (index >= 0) {
e = &st->index_entries[index];
av_assert1(e->timestamp >= target_ts);
pos_max = e->pos;
ts_max = e->timestamp;
pos_limit = pos_max - e->min_distance;
av_log(s, AV_LOG_TRACE, "using cached pos_max=0x%"PRIx64" pos_limit=0x%"PRIx64
" dts_max=%s\n", pos_max, pos_limit, av_ts2str(ts_max));
}
}
pos = ff_gen_search(s, stream_index, target_ts, pos_min, pos_max, pos_limit,
ts_min, ts_max, flags, &ts, avif->read_timestamp);
if (pos < 0)
return -1;
/* do the seek */
if ((ret = avio_seek(s->pb, pos, SEEK_SET)) < 0)
return ret;
ff_read_frame_flush(s);
ff_update_cur_dts(s, st, ts);
return 0;
}注意avid->read_timestamap mpegts 将通过mpegts_get_dts 通过时间戳查询pos
cpp
AVInputFormat ff_mpegts_demuxer = {
.name = "mpegts",
.long_name = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
.priv_data_size = sizeof(MpegTSContext),
.read_probe = mpegts_probe,
.read_header = mpegts_read_header,
.read_packet = mpegts_read_packet,
.read_close = mpegts_read_close,
.read_timestamp = mpegts_get_dts,
.flags = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
.priv_class = &mpegts_class,
};
cpp
static int64_t mpegts_get_dts(AVFormatContext *s, int stream_index,
int64_t *ppos, int64_t pos_limit)
{
MpegTSContext *ts = s->priv_data;
int64_t pos;
int pos47 = ts->pos47_full % ts->raw_packet_size;
pos = ((*ppos + ts->raw_packet_size - 1 - pos47) / ts->raw_packet_size) * ts->raw_packet_size + pos47;
ff_read_frame_flush(s);
if (avio_seek(s->pb, pos, SEEK_SET) < 0)
return AV_NOPTS_VALUE;
while(pos < pos_limit) {
int ret;
AVPacket pkt;
av_init_packet(&pkt);
ret = av_read_frame(s, &pkt);
if (ret < 0)
return AV_NOPTS_VALUE;
if (pkt.dts != AV_NOPTS_VALUE && pkt.pos >= 0) {
ff_reduce_index(s, pkt.stream_index);
av_add_index_entry(s->streams[pkt.stream_index], pkt.pos, pkt.dts, 0, 0, AVINDEX_KEYFRAME /* FIXME keyframe? */);
if (pkt.stream_index == stream_index && pkt.pos >= *ppos) {
int64_t dts = pkt.dts;
*ppos = pkt.pos;
av_packet_unref(&pkt);
return dts;
}
}
pos = pkt.pos;
av_packet_unref(&pkt);
}
return AV_NOPTS_VALUE;
}未完待续